Typical examples of the transmission assembly for vehicles of the type set forth above are disclosed in U.S. Pat. Nos. 5,046,994 and 5,142,940. In each of the transmission assemblies disclosed in these U.S. patents, there is provided a hydrostatic transmission for changing the traveling speed of vehicle non-stepwisely. A fluid motor of the hydrostatic transmission is mounted on a front surface of a transmission casing, and the input of vehicle-driving power into the transmission casing is made from the motor shaft through a bevel gearing. The transmission assembly disclosed in U.S. Pat. No. 5,064,994 is further fashioned such that two-stage speed-change transmission is achieved in the transmission casing. For this, a drive shaft which extends within the transmission casing laterally of the casing has thereon a freely rotatable, hollow input shaft to which power is transmitted from the motor shaft set forth above. The transmission casing further includes in it an intermediate shaft, extending parallel to the drive shaft, and a speed-reducing gear train which is operable to transmit rotation of the input shaft to the drive shaft through the intermediate shaft. A first speed rotation of the drive shaft is attained by obtaining the operated condition of the speed-reducing gear train by means of a clutch member which is slidably but non-rotatably mounted on the drive shaft, whereas a second speed rotation of the drive shaft is attained by coupling this drive shaft directly to the input shaft using the clutch member set forth above. It is fashioned that the rotation of drive shaft is transmitted to the differential casing of a differential gearing with a reduced speed of rotation. On the other hand, the transmission assembly disclosed in U.S. Pat. No. 5,142,940 is fashioned such that power is transmitted from the motor shaft directly to a drive shaft, extending laterally of the transmission casing, and then to a differential casing through a speed-reducing gearing so that no speed-change transmission is made within the transmission casing.
Even in a transmission assembly including a hydrostatic transmission which permits a non-stepwise change of the traveling speed of vehicle, it is desirable to transmit the rotation of motor shaft of the hydrostatic transmission to a differential gearing and wheel axles with a largely reduced speed of rotation. This is because the rotational speed of motor shaft is relatively unstable at a low speed range of the hydrostatic transmission so that it is desirable to use this transmission at its higher speed range for assuring a stable traveling speed of vehicle. In this case, it is also desirable to construct the transmission assembly such that it permits not only a speed reduction but a stepwise change of two or more change stages so that two or more speed ranges of vehicle can be set at the transmission assembly. Of the transmission assemblies shown in the two U.S. patents referred to above, the one disclosed in U.S. Pat. No. 5,064,994 is superior in this point. This transmission assembly includes, however, relatively many gears and the additional intermediate shaft so that a relatively large manufacturing cost is required. Further, the additional intermediate shaft and the speed-reducing gear train associated therewith will enlarge the scale of transmission assembly.
From this, it may be considered to employ a speed-change gear mechanism known from, for example, JP,U No. 1-167921 which permits to transmit the rotation of a drive shaft, disposed laterally within the transmission casing, to a differential gearing with two or more reduced speeds of rotation. The known mechanism is, however, fashioned such that gears of the driving side of speed-change gear trains are rotatably mounted on the drive shaft so as to be coupled selectively to the drive shaft by clutch means. This clutch means prevents gears of the driving side from being reduced in diameter so as to highten speed-reducing ratios at the change mechanism. Consequently, the known mechanism does not permit a large reduction of rotational speed at the transmission path between the drive shaft and differential gearing. In the transmission assembly shown in the Japanese publication referred to above, speed-reducing gear trains are further provided between left and right output shafts of the differential gearing and left and right wheel axles for assuring a large reduction of rotational speed. In a speed-change gear mechanism which also functions as a speed-reducing mechanism, it is preferred to associate a shifting clutch mechanism to gears of the driven side, which are driven to rotate at reduced speeds of rotation, for avoiding shock caused during a shifting operation and for protecting elements of the mechanism from being damaged. The known mechanism is disadvantageous from this respect, too.
Accordingly, a primary object of the present invention is to provide a novel transmission assembly for vehicles in which a speed-change mechanism which performs a speed-change transmission from a drive shaft, extending laterally within a transmission casing, to a differential gearing with largely reducd speed ratios is provided in a cost-saving, scale-reducing and shock-avoiding manner.